I. Field of the Invention
The present invention relates to cationic electrodepositable coating compositions comprising a hydroxyl group-containing cationic resin, a blocked polyisocyanate curing agent, and an organotin catalyst; to methods of preparing such compositions; and to methods for applying such compositions.
II. Technical Considerations
The application of a coating by electrodeposition involves deposition of a film-forming composition to an electrically conductive substrate under the influence of an applied electrical potential. Electrodeposition has gained prominence in the coating industry because in comparison with non-electrophoretic coating methods, electrodeposition provides higher paint utilization, excellent corrosion resistance and low environmental contamination. Early attempts at commercial electrodeposition processes used anionic electrodeposition where the workpiece to be coated serves as the anode. However, cationic electrodeposition has become increasingly popular and today is the most prevalent method of electrodeposition.
Many cationic electrodeposition compositions in use today are based on active hydrogen-containing resins derived from a polyepoxide and a capped or blocked polyisocyanate curing agent. Typically, these cationic electrodeposition compositions also contain organotin catalysts to lower the temperature at which the blocking agent is released from blocked polyisocyanate and to activate cure of the electrodeposition composition.
Common organotin catalysts include dialkylltin oxides, for example, dibutyltin oxide, dioctyltin oxide and dimethyltin oxide, and derivatives thereof, such as dibutyltin dicarboxylates and dibutyltin mercaptides. Although effective to some degree in promoting cure of the electrodeposition composition, the use of such catalysts in cationic electrodeposition compositions can present several drawbacks. For example, most of the common dialkyltin oxides are high melting, amorphous solid materials which must be introduced into the composition in the form of a catalyst paste prepared by dispersing the solid catalyst into a pigment wetting resin under extremely high shear conditions. Preparation of stable catalyst pastes can be very costly and time intensive.
Further, it has been noted that some of the aforementioned organotin catalysts can cause a multitude of surface defects in the cured electrodeposited coating composition. For example, dibutyltin oxide dispersions can flocculate in the electrodeposition bath, resulting in oversized dibutyltin oxide agglomerates or particles which can settle in areas of the electrodeposition tank where agitation is poor. This flocculation phenomenon constitutes a loss of catalyst from the coating composition resulting in poor cure response. Moreover, the flocculate particles can settle in the uncured electrodeposited coating causing localized “hot spots” or pinholes in the surface of the cured coating. Also, electrodeposition bath stability can be adversely affected with the use of some organotin catalysts. It has been observed that soft, floating foams can form from a mixture of organotin catalyst, polyisocyanate curing agent and microscopic air bubbles.
Known in the art for use as catalysts in cationic electrodepositable coating compositions are the condensation products of dialkyltin oxides, such as dibutyltin oxide, and hydroxyl compounds such as aliphatic alcohols, alkanolamines, and phenols. These catalysts purportedly are storage stable and the organophilic molecule segments thereof enable them to stay in the resinous phase and, thus, in the dewatered film.
Also, known in the art for use as catalysts in cationic electrodepositable coating compositions are dialkyltin aromatic carboxylic acid salts prepared by reacting a dialkyltin oxide, such as dibutyltin oxide, with an aromatic carboxylic acid. Such catalysts are said to be more compatible with the resinous binder system, provide improved bath stability, and yield a coated film free from defects such as cratering and seeding. Further, a catalytic effect can be observed at lower temperatures than with the corresponding dialkyltin oxides, and cured films have improved corrosion resistance. Such catalysts can also be used in conjunction with bismuth and/or zirconium compounds to provide a cationic electrocoating composition having superior corrosion resistance, low-temperature curability and good electrodeposition bath stability.
Carboxylic acid and mercapto derivatives of dialkyltin oxides, for example diorganotin bis-carboxylates and diorganotin bis-mercaptides are also known in the art as catalysts for cationic electrodepositable coating compositions. The diorganotin bis-carboxylates can be prepared by reacting a dialkyltin oxide with the appropriate hydroxy-carboxylic acid or mercaptan. It is alleged that the use of such compounds as catalysts in cationic electrodepositable compositions can prevent problems associated with catalyst volatility and provide emulsion stability by becoming chemically bound via hydroxy-functionality to one or more of the composition components. However, with lower carboxylic acid derivatives, such as acetate, formate and laurate, the organotin derivative can hydrolyze and form the corresponding diorganotin oxide precipitate. The liberation of these low molecular weight carboxylic acids also can lower the throwpower of the electrodepositable composition, and can increase corrosion of the anode. Cautious incorporation of higher carboxylic acid derivatives such as dibutyltin dioleate, provide electrodeposition coating compositions with improved stability, however free acid from the at least partial hydrolysis of these organotin compounds can negatively affect coating performance when applied over galvanized steel substrates. It has been noted that with these higher carboxylic acid derivatives, the higher acid can remain in the film after curing, and can migrate to the zinc-electrodeposition coating interface causing adhesion loss and poor corrosion resistance.
Alkyltin diacetyl acetonates, for example dibutyltin diacetyl acetonate, have been employed as catalysts for curing components of cationic electrodeposition coatings containing a blocked polyisocyanate curing agent. Such catalysts typically are added in solution to a blend of an epoxy amine adduct and a blocked polyisocyanate curing agent, prior to dispersion into water. The alkyltin diacetyl acetonate catalysts purportedly are readily dispersed and remain dispersed in aqueous electrocoating baths. The art teaches that such materials are hydrolytically stable for extended periods of time under conventional electrocoating conditions. However, in practice, it is known in the art that some hydrolysis can occur, resulting in reformation of the dialkyltin oxide and the acetyl acetonate. The acetyl acetonate can react readily with any primary amine present in the cationic composition as a result of the reaction of a ketimine-containing compound with epoxy groups of a main film-forming resin, thereby forming high molecular weight species. In such instances, coating appearance can be adversely affected due to the presence of dialkyltin oxide precipitates and particles of the high molecular weight species resulting from reaction of primary amine and acetyl acetonate.
Triorganotin compounds are known for use as catalysts in electrodepositable coating compositions comprised of an active hydrogen-containing resin and a blocked polyisocyanate curing agent. For example, it is known to use triorganotin compounds such as bis (tributyltin) oxide, bis (trioctyltin) oxide, bis (tributyltin) sulfide, and bis (trioctyltin) adipate, which preferably are in liquid form at room temperature. These materials are easily incorporated into the electrodepositable composition and have good catalytic activity even at relatively low levels and at temperatures below 150° C. Such triorganotin compound, however, have been observed to have poor cure response when used in conjunction with resinous components having phenolic hydroxyl groups. Moreover, some trialkyltin compounds, for example, tributyltin compounds, are known to be volatile at typical curing temperatures. Also, some trialkyltin compounds can be toxic. Further, many triorganotin compounds typically have the disadvantage of high cost.
In view of the foregoing, it would be advantageous to provide a cationic electrodepositable coating composition containing an organotin catalyst which overcomes the problems encountered with prior art compositions containing such catalysts as discussed above. These problems are solved by the electrodepositable compositions of the present invention wherein the organotin catalyst can be incorporated into the resinous phase without the necessity of a grinding or milling operation. Such compositions demonstrate improved storage stability and cure response at lower cure temperatures, without compromising cured film appearance and performance properties.